Roof Truss Repair and Replacement for London Victorian Terraces: Cut Rafter, Attic Truss, and Structural Assessment

THE ROOF STRUCTURE OF A LONDON VICTORIAN TERRACE IS TYPICALLY 100 TO 150 YEARS OLD — DURING WHICH TIME IT HAS BEEN SUBJECT TO VARYING MOISTURE EXPOSURE, MULTIPLE ROOF COVERING REPLACEMENTS (WITH CHANGES IN LOADING), ALTERATIONS (PARTICULARLY REMOVAL OF INTERNAL LOAD-BEARING WALLS THAT MAY HAVE SUPPORTED PURLIN STRUTS), AND GENERAL FATIGUE.

TIMBER DECAY (WET ROT AND DRY ROT): THE MOST COMMON STRUCTURAL DEFECT IN CUT ROOFS OF LONDON VICTORIAN TERRACES: CAUSES: LONG-TERM MOISTURE INGRESS (FROM FAILED FLASHING, FAILED TILE/SLATE JOINTS, BLOCKED GUTTERS AND OVERFLOWS, OR FAILED RIDGE/VALLEY POINTING) THAT CREATES PERSISTENTLY WET TIMBER CONDITIONS — MOISTURE CONTENT ABOVE 20% ENABLES WET ROT FUNGI (CONIOPHORA PUTEANA) AND ABOVE 28% ENABLES DRY ROT (SERPULA LACRYMANS). LOCATIONS: WALL PLATE (THE HORIZONTAL TIMBER BEDDED ON TOP OF THE PARTY WALL AND EXTERNAL WALL — DIRECTLY IN THE MOISTURE PATH FROM FAILED GUTTERS OR OVERFLOWING VALLEY GUTTERS), RAFTER FEET (THE LOWEST 300-600MM OF EACH RAFTER — AT THE EAVES ABOVE THE WALL PLATE — EXPOSED TO DRIVING RAIN AND GUTTER OVERFLOW), VALLEY RAFTERS (IF PRESENT — AT THE JUNCTION OF TWO ROOF SLOPES — HIGH MOISTURE EXPOSURE FROM VALLEY GUTTER FAILURE), AND THE FIRST 1-2M OF THE RIDGE BOARD AT EACH END (EXPOSED TO MOISTURE FROM FAILED CHIMNEY STACK FLASHING OR FAILED RIDGE TILING).

RIDGE SPREAD (WALL SPREAD): A VERY COMMON DEFECT IN LONDON VICTORIAN TERRACE CUT ROOFS — OCCURS WHEN THE HORIZONTAL THRUST OF THE RAFTERS (THE FORCE THAT THE RAFTER EXERTS OUTWARDS ON THE WALL PLATE AND WALL BELOW AS IT IS LOADED VERTICALLY BY THE ROOF COVERING WEIGHT) IS NOT ADEQUATELY RESISTED — PARTICULARLY WHERE: COLLARS HAVE BEEN REMOVED (OFTEN DURING LOFT CONVERSIONS — THE COLLAR, WHICH TIES OPPOSING RAFTERS AND RESISTS SPREAD, IS FREQUENTLY REMOVED TO IMPROVE HEADROOM — WITHOUT ADEQUATE ALTERNATIVE PROVISION). CEILING JOIST TIES HAVE BEEN REMOVED (THE CEILING JOISTS, WHICH CONNECT THE FOOT OF OPPOSING RAFTERS, ACT AS TIES RESISTING THE HORIZONTAL THRUST — THEIR REMOVAL IN ANY FLOOR OPENING, LOFT HATCH, OR STAIRCASE ALTERATION THAT CUTS THE CEILING JOIST INTERRUPTS THIS TIE). DETECTION: CRACK AT JUNCTION BETWEEN EXTERNAL WALL AND ROOF SLOPE (EXTERNALLY — TYPICALLY VISIBLE AT THE JUNCTION BETWEEN THE RENDER OR BRICK FACE AND THE SOFFIT/VERGE); DOORS AND WINDOWS ON THE UPPER FLOOR STICKING OR DROPPING (INDICATING GENERAL MOVEMENT OF THE WALL BELOW THE RAFTER FOOT); BOWED OUTWARD WALL FACE BELOW EAVES LEVEL.

PURLIN SAG: WHERE THE PURLIN (THE MAIN HORIZONTAL MEMBER CARRYING THE RAFTERS AT MID-SPAN) SAGS UNDER LOAD — TYPICALLY BECAUSE ITS SUPPORTING STRUT HAS BEEN REMOVED (WHEN THE LOAD-BEARING WALL BELOW WAS OPENED UP OR REMOVED), BECAUSE THE PURLIN-TO-STRUT JUNCTION HAS DECAYED, OR BECAUSE THE PURLIN ITSELF HAS INSUFFICIENT SECTION FOR THE SPAN. DETECTION: VISIBLE SAG IN THE ROOF SLOPE (EXTERNALLY — A DIP OR HOLLOW IN THE ROOF SLOPE MIDWAY BETWEEN EAVES AND RIDGE — OFTEN MOST VISIBLE AS A SHADOW LINE OR DISRUPTION IN THE TILING PATTERN WHEN VIEWED AT AN ANGLE FROM GROUND LEVEL OR FROM OPPOSITE BUILDINGS).

A PROPER STRUCTURAL ASSESSMENT OF THE CUT ROOF OF A LONDON VICTORIAN TERRACE MUST BE CARRIED OUT BY A STRUCTURAL ENGINEER (MEMBER OF THE INSTITUTION OF STRUCTURAL ENGINEERS — MINSTRUCT OR CENG — OR MEMBER OF THE INSTITUTE OF CIVIL ENGINEERS — MICE OR CENG) BEFORE ANY SIGNIFICANT REPAIR OR ALTERATION IS SPECIFIED.

ROOF STRUCTURE SURVEY SCOPE: A THOROUGH ROOF STRUCTURE SURVEY FOR A LONDON VICTORIAN TERRACE CUT ROOF SHOULD COVER: RAFTER SIZES AND SPACINGS (MEASURE ACTUAL CROSS-SECTIONS — ORIGINAL TIMBER SECTIONS IN VICTORIAN CONSTRUCTION WERE OFTEN SMALLER THAN MODERN STANDARDS — STRUCTURAL ENGINEER TO VERIFY ADEQUACY FOR CURRENT AND PROPOSED LOADING). PURLIN SIZES, SPANS, AND SUPPORT POINTS (IDENTIFY ALL STRUT POSITIONS — VERIFY STRUTS ARE BEARING ON LOAD-BEARING WALLS OR ADEQUATE STRUCTURE BELOW — ANY ALTERATION TO THE STRUCTURE BELOW A PURLIN STRUT WITHOUT REPLACEMENT SUPPORT IS A STRUCTURAL RISK). PRESENCE AND CONTINUITY OF COLLARS AND CEILING JOIST TIES (ESSENTIAL TO CONFIRM NO COLLARS OR TIES HAVE BEEN REMOVED WITHOUT ADEQUATE REPLACEMENT). TIMBER CONDITION (VISUAL INSPECTION PLUS PROBE WITH BRADAWL AT ALL HIGH-RISK LOCATIONS — WALL PLATE, RAFTER FEET, VALLEY RAFTERS, RIDGE ENDS — MOISTURE METER READINGS WHERE ACCESSIBLE). EAVES DETAIL (CONFIRM ADEQUATE BEARING OF RAFTER FEET ON WALL PLATE — IDENTIFY ANY NOTCHING OR CUTTING OF RAFTER FEET THAT REDUCES THE SECTION AT THE POINT OF MAXIMUM SHEAR FORCE). ROOF SLOPE (MEASURE PITCH — TYPICALLY 35-45° FOR A LONDON VICTORIAN TERRACE TILED/SLATED ROOF — CONFIRM ADEQUATE FOR THE PROPOSED COVERING AND BS 5534:2014 FIXING REQUIREMENTS FOR THE TILE WEIGHT AND EXPOSURE CATEGORY).

BOREHOLE INSPECTION (BORESCOPE): WHERE THE ROOF STRUCTURE IS NOT FULLY ACCESSIBLE FROM THE INTERIOR (NO LOFT HATCH, OR ROOF STRUCTURE WITHIN A SEALED LOFT SPACE), A BORESCOPE INSPECTION THROUGH A SMALL DRILLED HOLE IN THE CEILING (OR THROUGH A REMOVED CEILING TILE) ALLOWS VISUAL ASSESSMENT OF THE ROOF STRUCTURE WITHOUT FULL OPENING-UP. SURVEY FEE (STRUCTURAL ENGINEER — CUT ROOF INSPECTION): £500 TO £1,500 DEPENDING ON SCOPE AND ENGINEER.

WHERE INDIVIDUAL RAFTERS (OR SECTIONS OF RAFTERS) HAVE DECAYED OR BEEN DAMAGED, REPAIR IS USUALLY POSSIBLE WITHOUT REPLACING THE ENTIRE ROOF STRUCTURE — THE KEY APPROACHES ARE SISTERING (ADDING A NEW RAFTER ALONGSIDE THE OLD), EPOXY CONSOLIDATION AND FILLER (FOR LIMITED DECAY IN AN OTHERWISE SOUND RAFTER), AND STRUCTURAL SPLICING (BOLTED STEEL FLITCH PLATES).

SISTERING (THE MOST COMMON RAFTER REPAIR METHOD FOR LONDON VICTORIAN TERRACE CUT ROOFS): SISTERING INVOLVES INSTALLING A NEW FULL-DEPTH RAFTER ALONGSIDE THE EXISTING DECAYED RAFTER, FIXING IT TO THE SAME RIDGE BOARD AT THE TOP AND THE SAME WALL PLATE AT THE BOTTOM (OR TO A NEW SISTER WALL PLATE WHERE THE WALL PLATE ITSELF HAS DECAYED), AND BOLTING OR SCREWING THE NEW RAFTER TO THE OLD AT 600MM INTERVALS. SPECIFICATION: C24 STRENGTH CLASS TIMBER (TO BS 5268-2 / BS EN 338 — THE STRUCTURAL STRENGTH CLASS APPROPRIATE FOR RAFTER USE — MOST COMMONLY CLS-QUALITY TIMBER MARKED C24 FROM MERCHANT STOCK). SIZE: MATCH THE EXISTING RAFTER SIZE (TYPICALLY 50X100MM OR 50X125MM — OR UPSIZE IF THE STRUCTURAL ENGINEER SPECIFIES ADDITIONAL SECTION FOR THE LOAD). TREATMENT: PRESERVATIVE TREATMENT (VACSOL AZURE PRESSURE-TREATED, BS 8417:2011 USE CLASS 2 — FOR INTERNAL STRUCTURAL TIMBER ABOVE DPC LEVEL). FIXING: STAINLESS STEEL TIMBER SCREWS (SPAX OR EQUIVALENT — 6.0X180MM — AT 600MM CENTRES — DRIVEN THROUGH THE NEW RAFTER INTO THE OLD AT A SLIGHT ANGLE).

EPOXY CONSOLIDATION (FOR LIMITED RAFTER DECAY — WHERE THE OUTER SKIN HAS DECAYED BUT THE CORE REMAINS SOUND — CONFIRMED BY BRADAWL PROBE): RONSEAL HARDENER (CONSOLIDANT) APPLIED TO THE EXPOSED DECAYED ZONE — HARDENS AND STABILISES THE DECAYED TIMBER BEFORE EPOXY FILLER APPLICATION. RONSEAL HIGH PERFORMANCE WOOD FILLER (2-PART EPOXY) — FILLS THE EXCAVATED DECAYED ZONE — CURES TO STRUCTURAL HARDNESS. FOR PRINCIPAL STRUCTURAL MEMBERS (PURLINS, RIDGE BOARD): EPOXY REPAIR IS ONLY APPROPRIATE WHERE THE SECTION LOSS IS LESS THAN 20% OF THE CROSS-SECTIONAL AREA — BEYOND THIS, A STRUCTURAL ENGINEER MUST CONFIRM THE ADEQUACY OF THE REPAIRED SECTION AND MAY SPECIFY ADDITIONAL SUPPORT (SISTERED MEMBER OR STEEL PLATE).

THE PURLIN IS THE PRIMARY STRUCTURAL MEMBER OF THE CUT ROOF OF A LONDON VICTORIAN TERRACE — AND ITS REPAIR OR REPLACEMENT IS A SIGNIFICANT STRUCTURAL OPERATION THAT REQUIRES A STRUCTURAL ENGINEER'S SPECIFICATION AND BUILDING REGULATIONS APPROVAL UNDER PART A (STRUCTURE).

PURLIN REPAIR OPTIONS: SISTERED PURLIN (ADDING A NEW PURLIN ALONGSIDE THE EXISTING — RUNNING THE FULL LENGTH OF THE ROOF — FIXED TO THE EXISTING PURLIN WITH BOLTS AND CONNECTOR PLATES): MOST APPROPRIATE WHERE THE EXISTING PURLIN HAS DECAYED OR INSUFFICIENT SECTION BUT IS STILL IN POSITION AND PROVIDING SOME SUPPORT — THE SISTER PURLIN TAKES OVER THE FULL LOAD. SIZE: DETERMINED BY THE STRUCTURAL ENGINEER (DEPENDENT ON RAFTER SPAN, RAFTER SPACING, TILE/SLATE WEIGHT, SNOW LOAD, AND WIND LOAD — TYPICALLY 100X150MM OR 100X175MM C24 SAWN SOFTWOOD FOR A STANDARD VICTORIAN TERRACE ROOF). SUPPORT (PURLIN STRUTS — CRITICAL): THE PURLIN MUST BE SUPPORTED AT INTERMEDIATE POINTS BY STRUTS RUNNING DOWN TO LOAD-BEARING WALLS BELOW — WITHOUT ADEQUATE STRUTS, THE PURLIN WILL SAG. STRUT SPECIFICATION: 100X100MM MINIMUM SECTION (C24) — BEARING ONTO A SPREADER PAD ON THE WALL BELOW (100X38MM ACROSS THE FULL WIDTH OF THE WALL) TO DISTRIBUTE THE POINT LOAD.

STEEL FLITCH PLATE PURLIN REPAIR (WHERE THE PURLIN IS SEVERELY DECAYED OR WHERE A LARGER SPAN IS REQUIRED): STEEL FLITCH PLATE: A STEEL PLATE (TYPICALLY 10-12MM THICK MILD STEEL FLAT PLATE — CUT TO WIDTH OF PURLIN CROSS-SECTION) BOLTED BETWEEN TWO TIMBER MEMBERS (OR BOLTED TO THE FACE OF THE EXISTING PURLIN) WITH STAINLESS STEEL OR HOT-DIPPED GALVANISED HIGH-TENSILE BOLTS (M12 AT 450MM CENTRES) — THE COMPOSITE TIMBER-STEEL SECTION HAS A MUCH HIGHER MOMENT OF RESISTANCE THAN THE TIMBER ALONE — ALLOWING A LONGER SPAN OR REDUCED SECTION. FLITCH PLATE SPECIFICATION: STRUCTURAL ENGINEER DESIGN REQUIRED (MOMENT OF RESISTANCE, BOLT PATTERN, BEARING DESIGN AT SUPPORT POINTS).

WALL PLATE REPAIR OR REPLACEMENT: THE WALL PLATE (THE HORIZONTAL TIMBER BEDDED ON TOP OF THE PARTY WALL OR EXTERNAL WALL ON WHICH THE RAFTER FEET SIT): OFTEN DECAYED IN LONDON VICTORIAN TERRACES (PARTICULARLY AT THE ENDS — WHERE VALLEY GUTTERS OVERFLOW — AND BELOW LEAKING FLASHINGS). REPAIR: WHERE DECAY IS LIMITED — EPOXY CONSOLIDATION. REPLACEMENT: WHERE DECAY IS EXTENSIVE — WALL PLATE CAN BE REPLACED SECTION BY SECTION (NOT ALL AT ONCE — THE ROOF COVERING ABOVE MUST BE TEMPORARILY SUPPORTED OR CAREFULLY CONTROLLED DURING THE REPLACEMENT — PROPPING OF RAFTERS MAY BE REQUIRED). NEW WALL PLATE: 100X50MM OR 100X75MM SAWN SOFTWOOD (C24, PRESERVATIVE-TREATED, USE CLASS 2) — BEDDED IN LIME MORTAR ON TOP OF THE WALL — DPC BETWEEN WALL PLATE AND MASONRY (LEAD CORE DPC OR HYLOAD PITCH POLYMER DPC — TO PREVENT MOISTURE FROM THE MASONRY WETTING THE WALL PLATE).

WHERE A LONDON VICTORIAN TERRACE LOFT CONVERSION REQUIRES THE EXISTING CUT ROOF STRUCTURE TO BE REPLACED ENTIRELY (RATHER THAN ADAPTED — AS IN THE COMMON BEAM-AND-ROOM-IN-ROOF APPROACH), PREFABRICATED ATTIC TRUSSES (ALSO CALLED ROOM-IN-ROOF TRUSSES OR HABITABLE ROOF TRUSSES) FROM MITEK (THE DOMINANT UK SUPPLIER OF ENGINEERED TIMBER ROOF TRUSSES — PART OF BERKSHIRE HATHAWAY) OR GANG-NAIL (THE ORIGINAL BRAND NAME FOR PRESSED METAL PLATE TRUSSES — NOW TRADING AS POSI-JOISTS/MITEK) ARE INCREASINGLY USED.

ATTIC TRUSS (ROOM-IN-ROOF TRUSS) — WHAT IT IS: A FACTORY-MANUFACTURED TIMBER TRUSS (GANG-NAIL PRESSED METAL PLATE CONNECTOR JOINTS — C24 TIMBER) DESIGNED TO SPAN THE FULL WIDTH OF THE ROOF (FROM WALL PLATE TO WALL PLATE) WHILE PROVIDING A CLEAR HABITABLE SPACE WITHIN THE TRUSS PROFILE — THE TRUSS INCORPORATES VERTICAL MEMBERS AND HORIZONTAL BOTTOM CHORD (CEILING LEVEL) THAT DEFINE THE HABITABLE SPACE, WITH SLOPING TOP CHORDS (RAFTERS) ABOVE. SPAN: MITEK AND GANG-NAIL ATTIC TRUSSES AVAILABLE IN STANDARD SPANS FROM 6M TO 14M (CUSTOM SPANS ON REQUEST FROM MITEK UK OR GANG-NAIL PARTNERS). PITCH: AVAILABLE IN ANY PITCH FROM 30° TO 60° (MATCHING THE EXISTING VICTORIAN TERRACE PITCH). SUPPLY: MITEK UK (MITEK INDUSTRIES LTD, NATIONAL DISTRIBUTOR — TIMBER MERCHANTS WITH MITEK DESIGN CERTIFICATION); GANG-NAIL UK (INDEPENDENT TIMBER FRAME MANUFACTURERS USING THE GANG-NAIL PLATE SYSTEM). INSTALLATION NOTE FOR LONDON VICTORIAN TERRACES: ATTIC TRUSSES ARE MUCH HEAVIER THAN INDIVIDUAL CUT RAFTERS — LIFTING THEM INTO POSITION TYPICALLY REQUIRES A CRANE OR LIFTING APPARATUS — DIFFICULT IN THE CONFINED BACK GARDENS OF LONDON TERRACES — USUALLY LIFTED OVER THE RIDGE FROM SCAFFOLD ON THE STREET FRONT OR THE REAR GARDEN BOUNDARY — CRANE LICENCE MAY BE REQUIRED FOR STREET CRANE LIFT. THIS IS A MATERIAL CONSIDERATION IN THE DECISION BETWEEN ATTIC TRUSSES AND THE MORE COMMON BEAM-AND-ROOM-IN-ROOF STRUCTURAL APPROACH FOR LONDON TERRACE LOFT CONVERSIONS (WHICH USES INDIVIDUAL STEEL BEAMS AND C24 RAFTERS FITTED INDIVIDUALLY — MORE MANAGEABLE WITHOUT A CRANE).

ROOF STRUCTURE REPAIR IN A LONDON VICTORIAN TERRACE SPANS FROM MINOR EPOXY REPAIR OF A FEW RAFTER FEET TO MAJOR STRUCTURAL REPLACEMENT OF THE ENTIRE PURLIN AND WALL PLATE — AND COSTS VARY ACCORDINGLY.

STRUCTURAL SURVEY (STRUCTURAL ENGINEER — CUT ROOF INSPECTION AND REPORT): £500 TO £1,500 (DEPENDING ON SCOPE — INCLUDES WRITTEN REPORT AND REPAIR SPECIFICATION APPROPRIATE FOR BUILDING REGULATIONS SUBMISSION).

RIDGE BOARD REPAIR (LIMITED EPOXY — 1-2M SECTION — MATERIALS AND LABOUR): £150 TO £400.

RIDGE BOARD REPLACEMENT (FULL LENGTH — SCAFFOLD, TILE STRIP, REPLACE, RE-TILE): £1,200 TO £3,500 (FULL SCAFFOLD REQUIRED — MAJOR OPERATION — TILES MUST BE CAREFULLY STRIPPED AND REUSED OR REPLACED).

INDIVIDUAL RAFTER SISTERING (5 RAFTERS — MATERIALS AND LABOUR): £300 TO £800.

WALL PLATE REPLACEMENT (FULL LENGTH REAR SLOPE — SECTION BY SECTION — MATERIALS AND LABOUR — NO SCAFFOLD): £600 TO £1,500 (MATERIALS MODEST — LABOUR IS THE MAIN COST DUE TO DIFFICULTY OF ACCESS AND NEED TO PROP RAFTERS DURING REPLACEMENT).

PURLIN REPAIR (SISTER PURLIN FULL LENGTH + NEW STRUTS + SPREADER PADS — MATERIALS AND LABOUR): £1,500 TO £4,000.

FULL ROOF STRUCTURE REPLACEMENT (CUT ROOF — MATERIALS C24 TIMBER + LABOUR — EXCLUDES TILES, SCAFFOLD): £6,000 TO £15,000 (A MAJOR PROJECT — COMPARABLE IN COST TO A SIGNIFICANT LOFT CONVERSION STRUCTURE).

BUILDING REGULATIONS: ANY STRUCTURAL REPAIR OR ALTERATION TO THE ROOF STRUCTURE OF A LONDON VICTORIAN TERRACE (INCLUDING PURLIN REPAIR, WALL PLATE REPLACEMENT, OR RAFTER STRENGTHENING BEYOND LIKE-FOR-LIKE REPAIR) REQUIRES BUILDING REGULATIONS APPROVAL UNDER PART A (STRUCTURE) — EITHER FULL PLANS SUBMISSION OR BUILDING NOTICE TO THE LOCAL AUTHORITY BUILDING CONTROL (LABC) OR TO AN APPROVED INSPECTOR — WITH STRUCTURAL ENGINEER CALCULATIONS SUPPORTING THE SPECIFICATION.